1995 Pontiac Bonneville Stumbling during acceleration

My Bonneville will run fine but stumbles when going up a hill or during acceleration while cruising at highway speeds. If I go into passing gear it will smooth out. Sometimes it gets so bad the fuel pump in the gas tank hums pretty lowd (can be heard over the A/C & radio). I checked the fuel pressure and it was fine. The fuel pump was changed a few years ago and tune-ups are done regularly (including air & fuel filter).